With the development of modern industry, the volume of gypsum in storage which is the by-product let out in processes of modern industry is on the increase with each passing day. For example, in the Qingshuitang industrial district of the Zhuzhou city in China, over 200,000 tons of heavy metal waste gypsum are produced annually up to 2013s, while there was no efficient method to utilize these waste gypsum comprehensively. There are nearly one hundred acres has been occupied. Even Xingjian River is polluted severely. As well as that is great waste of resource. Therefore, effective technologies of resourceful treatment and industrialization production are urgent to develop. Consequently, integrated utilization of the industrial by-product waste gypsum begins to be noticed and paid attentions, even some experts come up with the idea that nature gypsum the raw material can be replaced by the by-product gypsum to produce sulfuric acid.
Nowadays, the integrated utilization for waste gypsum has been applied to three aspects, primarily, as the following:
Firstly, the application in the field of architecture [Brick and Title World, 2008, (2): 23-2] that waste gypsum is directly used as gypsum plaster, gypsum wallboard, and gypsum blocks, primarily. Secondly, it is the utilization in the field of cement [China Building Material, 1995, (7): 27-2; Journal of Chemical Industry and Engineering, 2003, (3): 18-20; Cement, 2007, 8: 16-1; Study on Modified Phosphogypsum Used as Cement Setting Time Agent (dissertation), 2007; Cement and Concrete Research, 1989, 19(3): 377-384], in which waste gypsum is mainly recycled in the preparation of cement and cement retarder. Thirdly, the application in the field of agriculture, waste gypsum is treated as soil amendment and fertilizer, primarily, wherein the soil amendment is relied on the exchange interaction between waste gypsum and the sodium ions in sodium bicarbonate soil. While the reaction between waste gypsum and ammonium carbonate fertilizer is utilized to add the nutrition of sulfur to fertilizer.
As to the application of heavy metal waste gypsum in the field of architecture, the most difficult and most critical problem is that a certain amount of heavy metal is still residual in waste gypsum. So when it is directly used as construction material or admixture without removing heavy metal, this would lead to direct or potential heavy mental pollution. Therefore, the recycling method of the building material preparation can not meet the demand of the disposition of heavy metal waste gypsum. According to the Cement Kiln Co-processing Solid Waste Pollution Control Standards and the National Standard Cement Industry Emission Standard Atmospheric People's Republic of China implemented on March 1 this year, higher requirements to cement kiln co-processing solid waste would be put forward, as well as the preparation of sulfuric acid and cements, and cement-coagulation agents by heavy metal waste gypsum would be restrained strictly. As a result, the demand for the disposition of heavy metal waste gypsum can not be satisfied with the application in the field of cement. Then the application of heavy metal waste gypsum in the field of agriculture would be restricted in the process of extension and application for the existence of heavy metal. In conclusion, now, the demand for the disposition of heavy metal waste gypsum can not be satisfied with the comprehensive recycling method of waste gypsum, and we have no time to delay the research of the harmless method of recycling heavy metal waste gypsum.
There are affluent calcium and sulfur in waste gypsum. More and more attentions have been paid to the research for comprehensive application of calcium and sulfur, nowadays. Many scholars have studied the decomposition characteristics of waste gypsum in different atmospheres [Environmental Science and Technology. 2010, 12(33):144-148], with different reducing agents [J. Chem. Thermodynamics, 2013, 57: 39-45; Chemical Engineering Research and Design, 2011, 89: 2736-2741]. The study aimed to lower the decomposition temperature by mixing a certain amount of reducing agents or keeping it in the low oxygen atmosphere (reducing atmosphere or protecting atmosphere of nitrogen). Moreover many scholars have been trying to apply the study to industrial production. However, there are some crucial problems with the traditional, mainstream disposal method, as the following: (1) The sintering temperature is too high, and the atmosphere is too demanding; it has been proved that the temperature of the decomposition of gypsum is about 1600° C. according to traditional theory and practice, while the decomposition temperature will lower to 1100° C. in the protecting atmosphere of nitrogen with some reducing agents such as coal or lower oxygen reducing atmosphere. (2) As the decomposition happens in protecting atmosphere of nitrogen and reducing atmosphere, the concentration of sulfur dioxide reduces in tail gas, and then it is more difficult for sulfur dioxide to be collected, leading to the rise of the investment in environmental protection. (3) Difficulties in the extension of traditional disposal method, the lack of scientific theoretical research, rigorous demand for the production environment, and the uncontrollability of the reaction process. Moreover, the national standards can not be satisfied with the vast majority recycling products of waste gypsum. All of those result in the deficient utilization of waste gypsum. (4) The by-product sulfur is produced easily in the decomposition process, leading to problems such as scorification, agglomeration and jamming in facilities. Above all, the trend of waste gypsum resourceful utilization containing calcium and sulfur is to develop the decomposition technology of waste gypsum without reducing agents or protecting atmosphere.
In lead smelting industry, it needs a great amount of lime to be used as slag former. Meanwhile, it requires complete system for acid making and heavy metal disposal (smoke, dust, and wastewater). The sulfur dioxide smoke obtained in the process of heavy metal waste gypsum decomposition, can be used in the acid making system to realize the comprehension utilization of sulfur. And the product calcium oxide obtained in decomposition can be used as slag former for lead smelting. Moreover, heavy metal smokes and dust would be respectively disposed in corresponding systems for heavy metal to avoid the secondary pollution of heavy metal. In all, it is far-reaching that have the heavy metal waste gypsum recycled as the available resource for the slag former of lead smelting, avoiding secondary pollution of heavy metal waste gypsum. And the application will realize turning waste into wealth, which has well economic and remarkable environmental benefit.
The principal elements in the lead fuming furnace slag are Si, Ca, Fe and O. It also contains some heavy metals such as Zn, Pb, Cr, and Cu. The total amount of the heavy metals exceeds 5%. Direct application of the lead fuming furnace slag in building and road construction material will cause secondary pollution.
Geopolymer is a kind of three-dimensional network gel with amorphous and quasi crystalline characteristics, polymerized by silicon-oxygen and aluminum-oxygen tetrahedron polymerization. Geopolymer could substitute for ordinary cement in many applications. Compared to ordinary cement in the aspect of heavy metal solidification, geopolymer has the following advantages: (1) better early age strength and mechanical properties; (2) better acid or alkali resistance; (3) green energy-saving and environment friendly; (4) better heavy metals immobilization effect. Geopolymer has a kind of “quasi crystalline” structure composed by the cyclic molecules chain. The combination of the cyclic molecules forms a closed hollow space, which metal ion and other poisonous substance can be segmented and surrounded inside and heavy metal ion can be adsorbed by aluminum ion in the skeleton.